Poly(trimethylene terephthalate) fibers are revolutionary fibers that have both properties similar to nylon fibers, such as a soft touch, excellent elastic recovery and ready dyeability because of their low elastic modulus, and properties similar to poly(ethylene terephthalate) fibers such as wash-and-wear properties, dimensional stability and yellowing resistance; these characteristics are increasingly being used in applications such as clothing, carpets and the like.
Poly(trimethylene terephthalate) can be polymerized by the same process as poly(ethylene terephthalate) and poly(butylene terephthalate) which have a related chemical structure. That is, first terephthalic acid or a lower alcohol diester of terephthalic acid such as dimethyl terephthalate and trimethylene glycol (also referred to as 1,3-propanediol) may be subjected to the ester exchange reaction represented by chemical equation (1) while heating either without a catalyst or in the presence of a catalyst such as metal carboxylic acid salt, a titanium alkoxide or an organic acid, and then subjected to the polycondensation reaction represented by chemical equation (2) under reduced pressure using a catalyst such as a titanium alkoxide or an antimony oxide. EQU ROOC.o slashed.COOR+HOCH.sub.2 CH.sub.2 CH.sub.2 OH.fwdarw.HOCH.sub.2 CH.sub.2 CH.sub.2 OOC.o slashed.COOCH.sub.2 CH.sub.2 CH.sub.2 OH+ROH equation (1) EQU (R: --H or --CH.sub.3, .o slashed.: benzene ring with para-bonding) HOCH.sub.2 CH.sub.2 CH.sub.2 OOC.o slashed.OOCH.sub.2 CH.sub.2 CH.sub.2 OH.fwdarw.(OCH.sub.2 CH.sub.2 CH.sub.2 OOC.o slashed.CO).sub.n equation (2)
However, polymerization of poly(trimethylene terephthalate) is associated with a number of technical difficulties unlike poly(ethylene terephthalate) and poly(butylene terephthalate), and these problems have not yet been surmounted. Specifically, these technical problems may be largely classified into the three problems of whiteness, spinning stability and melt stability.
The problem of whiteness arises because yellow discoloration of the polymer during the polymerization stage results in discoloration of the fibers or fabric as well, thus impairing the product performance. The soft touch, excellent elastic recovery and easy care properties of poly(trimethylene terephthalate) fibers are expected to be advantageously used particularly in the fields of inner wear, panty stockings, sportswear, outer wear, etc. For commercial product development in these fields it is necessary for the fibers to have sufficiently high whiteness, so that they can be aesthetically colored with different light or dark colors. However, poly(trimethylene terephthalate) tends to undergo discoloration during the polymerization stage, and when colored polymers with poor whiteness are used for production of fibers and fabrics, the discoloration of dyed fiber products loses its clarity and the product value is notably diminished.
The problem of spinning stability arises because the abundant impurities in the polymer adversely affect the spinning stability. Specifically, the large amount of cyclic dimers as well as cyclic and linear oligomers that are present in the polymerization process for poly(trimethylene terephthalate) precipitate around the spinneret during spinning, and this creates the problem of more yarn breakage requiring a higher spinneret cleaning frequency (called the wiping period). The amount of cyclic dimers is especially high and constitutes the major cause of this problem.
The problem of melt stability occurs because the molten polymer lacks thermal stability, resulting in molecular weight reduction or discoloration of the polymer. The tendency toward molecular weight reduction means, particularly, that even if the molecular weight is increased at the polymer stage, the molecular weight will be reduced at the melt spinning stage. When this phenomenon occurs it becomes difficult to increase the tenacity of the fibers, and this has an adverse effect on the basic performance of the commercial product by lowering the tear strength and strength at break of the fabric product.
Although complete resolution of these three technical problems would give poly(trimethylene terephthalate) fibers with excellent whiteness, suitability for industrial production and sufficient tenacity, the prior art includes no knowledge of poly(trimethylene terephthalate) and fibers using it that satisfy all of these conditions.
Several publicly known techniques are known as methods for improving the whiteness and spinnability of poly(trimethylene terephthalate).
For example, in Japanese Unexamined Patent Publication (Kokai) No. 5-262862 there is disclosed a technique using a tin catalyst as a polymerization catalyst for improved whiteness. Upon investigation by the present inventors, however, the use of a tin catalyst results in a very high polymerization speed but the whiteness is instead inferior to using titanium alkoxides as catalysts. Zinc acetate has also been used as an ester exchange catalyst and tin catalysts have been used as polycondensation catalysts, but when such combinations are used for simple melt polymerization without solid state polymerization, the amount of cyclic dimers exceeds 3 wt %, which is unfavorable for the spinning stability. In the examples found therein, tridecyl phosphate is included at up to 500 ppm during the polymerization. Inclusion of such a long-chain compound presents disadvantages such as efflux of foam during the dyeing stage and a tendency to form dyeing spots. Furthermore, the use of tin catalysts and tridecyl phosphate has resulted in lower tenacity of the obtained fibers, making it difficult to exhibit tenacity of 3.5 g/d or greater.
An alternative has been proposed to the use of titanium catalysts as ester exchange reaction catalysts and antimony catalysts as polycondensation catalysts, for improved whiteness (Chemical Fiber International, Vol.45, pp.263-264, 1996). This document also touches on generation of by-products, indicating that poly(trimethylene terephthalate) can sometimes contain over 3% oligomers, which impurities become a problem in the spinning step and dyeing step. According to investigation by the present inventors, however, the use of antimony catalysts result in a lower polymerization speed, thus lengthening the time during which the polymer is exposed to high temperature and producing instead reduced whiteness. Furthermore, no concrete suggestion is provided in regard to reducing the oligomer content and improving the polymer melt stability in this process.
Japanese Unexamined Patent Publication (Kokai) No. 8-311177 teaches that poly(trimethylene terephthalate) with a limiting viscosity of 0.9 or greater, a b value (index for judging yellowing of the tip) of no greater than 10 and an oligomer content of no greater than 1 wt % can be obtained by subjecting poly(trimethylene terephthalate) obtained by a common method to solid state polymerization at 190-200.degree. C., for the purpose of reducing white powder produced at and around the spinning nozzle surface during the spinning step and to minimize yarn breakage. According to the examples in this publication, terephthalic acid and 1,3-propanediol are subjected to solventless ester exchange without using phosphorus compounds or cobalt compounds, and then tetrabutyl titanate (titanium butoxide) is added to prepare a prepolymer with a limiting viscosity of 0.70, after which solid state polymerization is carried out to obtain a polymer with a limiting viscosity of 1.02. When such polymers are melted, however, they undergo sudden thermal decomposition, lowering the molecular weight. Thus, highly polymerized polymers obtained by this process cannot give fibers with sufficient tenacity because of the viscosity reduction that occurs during the spinning stage.
In International Patent Application No. (WO)9823662 there is disclosed poly(trimethylene terephthalate) that is terminally masked with a hindered phenol portion to reduce the amount of acrolein produced upon heating in air, and a process for its production. In this known case, terephthalic acid, trimethylene glycol and an ester-forming monofunctional dicarboxylic acid with a hindered phenol portion are subjected to ester exchange reaction at high pressure in the presence of a triphenyl phosphate-based stabilizer, after which polycondensation reaction is carried out to obtain a polymer. However, the triphenyl phosphate-based stabilizer used here partially sublimates during the polycondensation reaction process which is carried out at high temperature in a vacuum, such that the elemental phosphorus content of the resulting polymer is significantly reduced. The polymer therefore has low melt stability, making it difficult to obtain high tenacity fibers. This publication also offers no suggestion regarding the issue of polymer melt stability conferring tenacity to the fibers. While the publication teaches that bis(3-hydroxypropyl)ether is contained in the polymer at about 4 mole percent (exceeding 2 wt %), such a polymer has considerably low heat resistance and light fastness and therefore cannot be used for production of clothing fibers. In addition, since this publication concerns a technique of conducting ester exchange reaction under high pressure, for industrial applications it requires large equipment that can withstand high pressure, and therefore the technique has an economic drawback.
Japanese Unexamined Patent Publication (Kokai) No. 9-195142 discloses fibers using poly(trimethylene terephthalate) with a limiting viscosity of 0.92, obtained by ester exchange reaction of terephthalic acid and 1,3-propanediol followed by polycondensation at 250.degree. C., and the examples include one where fibers with a tenacity of 5.2 g/d and an elongation of 41% are used as core filaments of multicomponent fibers. However, it is difficult to use this publicly known technique to overcome the problems of polymer whiteness and poor melt stability, for stable industrial production of fibers with a practical level of strength. This is because even when fibers having such tenacity are obtained, the low polycondensation temperature of 250.degree. C. means that the polymer will contain over 3 wt % of cyclic dimers, so that fluff and yarn breakage becomes more frequent and the resulting filaments have low uniformity.